Viton vs Nitrile O-rings...

[Cave Bum]

I'm in aerospace, for oxygen service Viton is used.

Well, no, not actually. In some applications at some times that is done, however there are a variety of materials used up to and including the silver-jacketed O-rings specified in some NASA applications.

As a mechanical engineer who has specified oxygen cleaning to US Air Force specifications, let me debunk a couple of the manufacturers myths here. First, many (quote-unquote) scuba "experts" will insist that viton O-rings are an absolute must especially for regulators used with mixes above 40% O2. This is not necessarily so. Viton has a slightly lower rating as a fuel then Buna-N, but your fuel rating is not the only issue. You have to look at the “fire triangle”. You need a fuel, and oxidizer and an ignition source for combustion. For the most part we control this in scuba equipment a little by reducing the fuel source but mostly by eliminating the ignition source. It is notable here to see that the one plastic piece that sees the release of high pressure gas and so is most exposed to the possibility of friction heating is... THE HIGH PRESSURE SEAT and they don't make that in a different material. O-rings that see low pressure are not near as much of a risk as is the high pressure seat.

In my regulators that I rebuild FOR MYSELF I use Buna-N O-rings. When working on customer equipment I always use the industry specified components for liability reasons.

Now having said above I don't worry about the fuel rating of Buna-N, I do worry about the fuel rating of petroleum products in tanks, either from dirty compressors or from cleaning solvents. I've had (quote-unquote) scuba “experts” tell my customers that there is no need to clean the tank in tanks we are going to be doing partial pressure mixing because the tanks only sees pure O2 at atmospheric pressure, that they just needed to clean the manifolds. My answer to this (besides yelling and swearing at the “expert”) is to remind the customer the agency that knows the most about handling oxygen is... NASA. Let’s look at their track record on handling oxygen as it relates to accidents. They burned up an Apollo capsule on the launch pad with a 100% oxygen environment at atmospheric pressure (14.72 PSI), the blew up an oxygen tank on the way to the moon on Apollo 13 at below atmospheric pressure (10 PSI) and the they blew up the Challenger Space Shuttle by burning a hole in the oxygen tank at well below atmospheric pressure (60,000 ft is what, maybe 8 PSI). All three of these tragic accidents were from introducing an ignition source to complete the fire triangle when fuel was present. In partial pressure blending we put 100% oxygen into the scuba tank then top with air to make Nitrox. If you open a valve too fast you can get friction heating past the valve seat in the tank valve. If I'm the one operating the valve, I don't want any fuel with a rating as high as petroleum products present if that accidentally happens.

So O2 cleaning of the regulator for nitrox percentages below 40% isn't necessary, but O2 cleaning a tank is.

One note that others have brought up when I’ve post this before, pre-blended Nitrox doesn't require tank cleaning. In North Central Florida, most of the fill stations are selling pre-blended Nitrox. That is not the case in many other areas of the country and around the world.

 

[Rick Inman]

Nice post, Cave Bum.

I O2 clean my tanks and regs (I do PP blending in my basement, but sometimes top at the shop). I used viton, but am not religious about it. However, because it is virtually impossible to prevent tanks and regs from acquiring minute particles of fuel to burn (or at least, knowing for a fact that they haven't), I fill my clean tanks as if they are dirty. IOW, slowly. Slowly with 100% - slow initial turning of the valve, slow filling. If I'm toping, once the mix is at 50% or so, I'm not afraid to crank it up a bit.

Bottom line, I keep my stuff as clean as I can, but then fill O2 as if it's all dirty.

 

[Cave Bum]

Nice post, Cave Bum.

I O2 clean my tanks and regs (I do PP blending in my basement, but sometimes top at the shop). I used viton, but am not religious about it. However, because it is virtually impossible to prevent tanks and regs from acquiring minute particles of fuel to burn (or at least, knowing for a fact that they haven't), I fill my clean tanks as if they are dirty. IOW, slowly. Slowly with 100% - slow initial turning of the valve, slow filling. If I'm toping, once the mix is at 50% or so, I'm not afraid to crank it up a bit.

Bottom line, I keep my stuff as clean as I can, but then fill O2 as if it's all dirty.

Rick,

I remember reading, but unfortunately I forget where, about scuba tanks used for oxygen being inspected and found to have a slightly burnt smell and small black streaks in them. The conclusion, if I remember correctly, was that they were getting mini-combustion during filling but that it never went catastrophic.

That scares the hell out of me. That's got to be right on the ragged edge...

Bruce

 

[Rick Inman]

Rick,

I remember reading, but unfortunately I forget where, about scuba tanks used for oxygen being inspected and found to have a slightly burnt smell and small black streaks in them. The conclusion, if I remember correctly, was that they were getting mini-combustion during filling but that it never went catastrophic.

That scares the hell out of me. That's got to be right on the ragged edge...

Bruce

The other day I was filling (decanting) my 20cf with O2, slowly, and when I came back to check on it, it was very hot. Scared me into draining the tank and looking inside, but I saw nothing unusual. It was outside in the sun at the time of filling, but still, it shouldn't have been that hot. Humm......

 

[fppf]

Bruce
I agree there are many O2 safe materials. But you will never see any Buna in any of our products. We don't even have them in stock. A viton neck oring is still less then 50 cents, I don't think its really an issue.

 

[skdvr]

So if you use Buna-N in your regs and tank valves and such do you check/change them more often or is it not a big deal at all so you just do it at what ever your normal interval is.

I kind of look at it as they are pretty unexpensive to replace once a year which is why I have all Viton in my O2 cleaned tanks, and on my one O2 cleaned reg. After reading this though I will not stress over it next time if I do not have the correct viton ring available...

Thanks
Phil

 

[Cave Bum]

Bruce
I agree there are many O2 safe materials. But you will never see any Buna in any of our products. We don't even have them in stock. A viton neck oring is still less then 50 cents, I don't think its really an issue.

The issue is how stiff it is. I've had sealing problems with viton.

As I said in my post, from a commercial standpoint, for liablity reasons, I would always use viton. From a personal standpoint, on my own equipment, I have no problem using buna-n in contact with pure O2.

This is the Do It Yourself Forum not a commercial 'how to design for NASA specifications' forum which is why I made that statement here. Even here, I wouldn't be so silly as to tell people it's okay to do that. I'm stating information so they can make their own decision.

Again, the nylon in the valve seat has a higher flamablity rating then the buna-n (if I remember correctly), and I know it's static elecectrical properties aren't good either, but they don't make valve seats in any other material.

Scuba tanks aren't designed for use with pure oxygen so, at best, what we're doing is making the best of an iffy situation.

In the fill station I worked in, we Haskelled oxygen out of the K bottles (or whatever size they were) into our oxygen bank and boosted it to 3300 psi. I was then filling customer bottles to 3000 psi. In the cave community many, if not most, clean and maintain their own tanks. I can just imagine the condition of some of those tanks I was filling... Now that's what's scarry...

 

[fppf]

Material has nothing to do with stiffness, Viton just like many other materials comes in many different durometers. I was under the belief that the valve seats for nitrox tanks where a teflon mix. But I have never seen anywhere published what the exact material is.

I do agree with regs, if your only using them to 40% Buna is fine. Most regs have many other parts that are custome to the mfgr made out of rubber based materials.

Just because its a do it yourself forum does not mean its a be a total cheap a** and penny pinch over a 50 cent oring. It should be a non issue because of the boat load of money your saving by doing it yourself. But I would still expect someone doing it there self to stay with industry practice. But we can have a difference in opinion, I have seen a lot of stuff that make me wonder how someone has not blown them selfs up as well.

 

[halocline]

I remember reading that viton was not great with regards to extrusion resistance or abrasion resistance. In some applications, like the HP o-ring in balanced piston regs, I use polyurethane or EDPM. My regs never see more than 40% O2 anyway.

If I were using 100% O2, I'd use a non balanced piston reg (SP MK2) and so would have no dynamic o-rings subjected to supply pressure. This would solve the dilemma of wanting to use a more extrusion-resistant o-ring in a high O2 percentage environment. With the various associated costs of learning to deco dive, all the extra equipment, O2 fills, etc, I doubt I'd be too worried about an extra dollar on o-ring costs.

 

[fppf]

Here is some good reading material. Piston regs don't really move that much. The stroke needed is only a few fractions of an inch.

http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf